1. Field of the Invention
The field of the invention is circuits useful in computers and transistor logic wherein digital signals are sequentially time processed onto a common bus line for removal.
2. Description of the Related Art
It is quite common in computer and other transistor logic design to have common bus lines which, as the name implies, is used in routing signals throughout a computer or other device for internal or external processing of digital signals residing on these bus lines. It is apparent also that signals which are being placed onto the bus lines and being removed from the bus lines must be properly time oriented. Specific digital signals are placed onto the bus lines only at certain times at one point so that they may be taken off the bus lines simultaneously at a distant point whereupon the bus lines are then available to receive other digital signals also to be taken off the lines at later designated times.
Normally, the signals which are placed upon the bus lines will come from many sources, such as an internal memory device, a microprocessor, a disc controller, communication adapter or the like. The signals are then shunted around the computer to other components or may even be pulled off the bus lines for transmission to a distant machine, such as a printer or another computer. All the inputs and outputs are time related.
To place signals upon the bus lines, multiple elements may be attached to the same bus line, commonly integrated circuit type line drivers are utilized, as well as manually operated electrical switches, termed dip switches. These dip switches usually have one side connected to a bus line and the other side grounded.
The dip switch, when closed, will very effectively ground that particular bus line. This ground is also normally the digital signal "0". It is apparent that the side of the dip switch opposite the bus line could also be connected to a voltage representative of the digital signal "1", normally +5 volt dc. In this latter case, with the dip switch closed, a "1" would at all times appear on the bus line. However, if that +5 volts were hard wired to the source of power, potential problems develop if one of the integrated circuit type line drivers also connected to the bus line were to output a ground.
Therefore, typically the side of the dip switch placed to ground or to the digital "1" voltage has interposed a current limiting resistor so as to eliminate this problem. Then, once the dip switch is set to an on or off, i.e., closed or open position, that digital signal will reside on the bus line continually until changed or until a stronger signal comes onto the bus line such as to swamp the signal from the dip switch.
This is essentially the case with line drivers, in that the output of the line driver connected to a bus line has such a very low output impedance such that were the line driver to output a voltage different then the voltage placed on that same bus line by a connected dip switch, there would be current flow between the line driver and the source of voltage or ground operably attached to the dip switch. In these cases, the voltage difference between the source and the ground resides across that current limiting resistor situated between the dip switch and the source of voltage or ground. The line drivers are so fabricated to easily absorb or deliver the current flowing through the dip switch resistor and thus to drop all the voltage across that resistor so that the output voltage of the line driver will be the voltage residing on the bus line.
In addition, connected to the bus lines are inputs to a second, or output, line driver in order that this second line driver receives the signal outputs of the bus lines and sends them on their way as needed.
Typically, in addition to the signal input and output lines, line drivers have an input termed the enable input. When the enable input to a line driver is asserted, the line driver commences its operation and the digital "1" or "0" on each of the inputs is outputted from each of the corresponding output lines with added substantial current delivering or absorbing capacity. The output signal continues so long as there is an input signal and the enable signal is present. At the time that the line driver is enabled, the output impedance of the line driver is very low, regardless of whether the output is a +5 volts dc (digital "1"), or 0 volts (digital "0"). At all other times when the line driver is not operating (enabled), each of its outputs have a very high output impedance such that the line driver presents no current burden upon the bus line to which it is connected.
It is to this commonly available system of placing a pre-selected digital signal on the bus line through a manually set dip switch that the present invention is directed.
Accordingly, it would be useful to allow more flexibility in placing digital signals from manually set dip switches onto bus lines so that it may be done in accordance with chosen timing. In addition, it would be useful to provide flexibility in changing a signal through a closed dip switch. More particularly, It is to this system of providing means for selectively altering the signal a closed dip switch places onto a bus line in accordance with the requirements of circuits to which the subject invention is directed.